The invention relates to a hydrostatic drive for driving a vehicle, in particular a commercial vehicle for use in forestry.
A hydrostatic drive according to the precharacterising clause of claim 1 is known, for example, from DE 195 24 189 C2. In this known hydrostatic drive, a hydraulic pump arranged with a hydraulic motor in a closed hydraulic circuit is driven by means of an internal combustion engine. Both the hydraulic pump and the hydraulic motor can be adjusted by an electrically controlled adjusting device in dependence on electrical control signals generated by an electrical control unit. The hydraulic motor drives the vehicle wheels of a vehicle axle via a change-speed gear and a differential gear. The disadvantage of this hydrostatic drive is that the drive takes place exclusively via a closed hydraulic circuit. The connection of the hydraulic motor to the hydraulic pump is relatively rigid, so that in the event of load alternation, in particular on uneven and rough terrain, spinning of the driving wheels frequently results. The mechanical differential gear has the disadvantage that, on rough terrain, a differential lock is necessary. When the differential gear is locked, however, slipping of the driving wheels when cornering inevitably occurs, resulting, particularly when used in forestry, in damage to the ground, which is to be avoided as far as possible.
Mobile implements used on rough terrain, in particular forestry implements, are frequently equipped with two or more driven vehicle axles, a vehicle joint allowing an angular offset between the driven vehicle axles in order to give the mobile implement the necessary off-road mobility. In this case, the driving of the vehicle axles is a particular problem. A known solution is to connect the vehicle axles by a cardan shaft. However, this involves a relatively high degree of structural complexity. The rigid coupling of the vehicle axles has the disadvantage of increasing soil erosion. Moreover, the maximum angular offset of the driven vehicle axles is limited by the cardan shaft.
Furthermore, it is known to drive the driven vehicle axles by means of separate hydraulic motors which are connected to a common hydraulic pump by a closed hydraulic circuit. Here, too, there is the disadvantage of undesirably high soil erosion due to the rigid hydraulic coupling. In mobile implements where the main function consists in the hydraulic operation of tools and where the hydrostatic drive is merely assigned an auxiliary function, it is known to operate the hydrostatic drive exclusively in an open hydraulic circuit. The disadvantage of this, however, is the low efficiency and the low torque which can be generated.
The object on which the invention is based therefore is to specify a hydrostatic drive in which both high efficiency and high torque are achieved with little damage to the ground.
The object is achieved by the characterising features of claim 1 in conjunction with the generic features.
According to the invention, a hydrostatic drive is provided which combines the advantages of a drive via an open hydraulic circuit with the advantages of a drive via a closed hydraulic circuit. According to the invention, two drive trains are therefore provided. In a first drive train a first hydraulic pump cooperates with a first hydraulic motor in a closed hydraulic circuit, whereas in a second drive train a second hydraulic pump cooperates with a second hydraulic motor in an open hydraulic circuit. The two hydraulic motors in this case drive different vehicle wheels, preferably different vehicle axles. A relatively high torque can be achieved with the drive train having a closed hydraulic circuit. In the drive train having the open hydraulic circuit the hydraulic motor is coupled relatively loosely to the hydraulic pump, enabling the rotational speed of the vehicle wheels driven by this drive train to adapt dynamically to the circumstances defined by the condition of the ground and the obstacles on the ground. As a result, account is taken of the fact that the rotational speeds of the two differently driven vehicle axles deviate from one another considerably when, for example, one of the two vehicle axles negotiates an obstacle. If the two vehicle axles are rigidly coupled to one another hydraulic or mechanically, as in the prior art, this would lead to slipping of one of the two vehicle axles, which inevitably causes damage to the ground. Through the relatively loose coupling, according to the invention, of the hydraulic motor to the hydraulic pump in one of the two drive trains, the rotational-speed differences described are for a short time equalised in the open hydraulic circuit. Since one of the vehicle axles is driven via a closed hydraulic circuit, however, a high torque can be generated nevertheless.
The subclaims relate to advantageous developments of the invention.
It is advantageous to arrange a control valve in the forward-flow line of the open hydraulic circuit. Preferably, two hydraulic motors driving opposite vehicle wheels of a vehicle axle are connected to the control valve of the open hydraulic circuit. The control valve then makes it possible for these hydraulic motors to be apportioned individual volumetric flows, so that given suitable control of the control valve a differential action can be achieved. At the same time, the apportioning of different volumetric flows to these two hydraulic motors also allows the vehicle to be steered. If the vehicle axles are not rigidly aligned with one another, as is customary in commercial vehicles for use in forestry, but rather the vehicle body is provided with a joint which allows a kink angle between the vehicle axles, then this kink angle can be increased or decreased in a specific manner by differential driving of the hydraulic motors connected to the control valve.
Furthermore, in the drive train having an open hydraulic circuit, the hydraulic motors provided there can be switched off during high-speed operation, for example on a forest road, so that the drive then takes place only via the closed hydraulic circuit. This has the advantage that the hydraulic motors operated in the open circuit do not have to run at a high rotational speed as well.
The open hydraulic circuit can, furthermore, be used at the same time for driving further hydraulic components, for example a saw, crane, etc. without an additional hydraulic pump being necessary for driving these further hydraulic components.